The effect of ATP and related compounds on spontaneous mechanical activity in the rat portal vein

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Pharmacology of adenosine receptors in the vasculature

Adenosine is widely distributed in mammals. One of the primary roles of adenosine within the cardiovascular system is to directly control the functions of both cardiac and vascular tissues. Recently, there has been considerable interest in the subclassification of adenosine receptors. Characterization of a heterogeneous population of receptors for adenosine could provide an opportunity for the development of novel compounds of therapeutic value. Adenosine is released from cells as a result of metabolism, and its release can be increased dramatically from cells that are metabolically stressed. This implies that adenosine can be released from a variety of cells throughout the body, as a result of increased metabolic rates, in concentrations that can have a profound impact on blood vessel function and, consequently, blood flow. It is recognized that the actions of this nucleoside on the vasculature are most prominent when oxygen demand is high and there is a reduction in oxygen tension at the site in question. Therefore, it is not surprising that adenosine has been shown to be an important regulator of blood vessel tone under hypoxic conditions. Furthermore, the activation of adenosine receptors on blood vessels can result in relaxation and/or contractions. The nature of the response subsequent to the activation of adenosine receptors is primarily dependent on the type of blood vessel involved and basal tone. This review will focus on the characterization of subtypes of adenosine receptors in blood vessels, as well as the effect of the stimulation of adenosine receptors on the peripheral circulation.

Haemodynamic effects of a selective adenosine A2A receptor agonist, CGS 21680, in chronic heart failure in anaesthetized rats

1. Recently we demonstrated that the administration of an A2A adenosine receptor agonist, CGS 21680, to anaesthetized rats with acute heart failure (1 h post-coronary artery ligation) resulted in an increase in cardiac output. In the present investigation, the effects of CGS 21680 on cardiac output, vascular resistance, heart rate, blood pressure and mean circulatory filling pressure (Pmcf) were investigated in anaesthetized rats with chronic heart failure (8 weeks post-coronary artery ligation). 2. Experiments were conducted in five groups (n = 6) of animals: sham-operated vehicle-treated (0.9% NaCl; 0.037 mL kg(-1) min(-1)) animals in which the occluder was placed but not pulled to ligate the coronary artery; coronary artery-ligated vehicle-treated animals; and coronary artery-ligated CGS 21680-treated (0.1. 0.3 or 1.0 microg kg(-1) min(-1)) animals. 3. Baseline blood pressure, cardiac output and rate of rise in left ventricular pressure (+dP/dt) were significantly reduced in animals with coronary artery ligation when compared to sham-operated animals. Coronary artery ligation resulted in a significant increase in left ventricular end-diastolic pressure, Pmcf and venous resistance when compared to sham-operated animals. 4. Administration of CGS 21680 at 0.3 and 1.0 microg kg(-1) min(-1) significantly (n = 6; P<0.05) increased cardiac output by 19+/-4% and 39+/-5%, and heart rate by 14+/-2% and 15+/-1%, respectively, when compared to vehicle treatment in coronary artery-ligated animals. Administration of CGS 21680 also significantly reduced blood pressure and arterial resistance when compared to coronary artery-ligated vehicle-treated animals. Infusion of CGS 21680 also significantly reduced venous resistance when compared to vehicle-treated coronary artery-ligated animals. 5. The results show that heart failure is characterized by reduced cardiac output, and increased left ventricular end-diastolic pressure, venous resistance and Pmcf. Acute treatment with CGS 21680 in animals with chronic heart failure decreased left ventricular end-diastolic pressure and increased cardiac output. This increase in cardiac output was the result of reduced arterial and venous resistances and increased heart rate.

Effects of CGS 21680, a selective A2A adenosine receptor agonist, on cardiac output and vascular resistance in acute heart failure in the anaesthetized rat

1. The effects of CGS 21680, a selective A2A adenosine receptor agonist, on cardiac output, blood pressure, mean circulatory filling pressure (Pmcf), arterial and venous resistances, heart rate and left ventricular end-diastolic pressure were assessed in rats with acute heart failure by means of coronary artery occlusion. 2. Animals (n=6 in each group) were divided into five groups: group I, sham-operated vehicle-treated (0.9% saline; 0.018 mL min(-1)); groups II-V, subject to coronary artery occlusion and treated with vehicle (0.9% saline; 0.018 ml min(-1)) and CGS 21680 (0.1, 0.3 and 1.0 microg kg(-1) min(-1)), respectively. Haemodynamic measurements were taken one hour after completion of surgery, ninety minutes after coronary artery occlusion (except in group I), and fifteen minutes after infusion of saline or CGS 21680. 3. Baseline haemodynamic measurements before occlusion were found not to differ significantly between the different groups of animals. However, after occlusion, cardiac output, rate of rise in left ventricular pressure (+ dP/dt) and blood pressure were significantly reduced when compared to corresponding values in sham-operated animals. In addition, occlusion of the coronary artery resulted in a significant elevation in venous resistance, Pmcf and left ventricular end-diastolic pressure as compared to corresponding values in sham-operated animals. 4. Infusion with CGS 21680 at the highest dose significantly reduced blood pressure, arterial resistance and left ventricular end-diastolic pressure when compared to occluded vehicle-treated animals (group II). Administration of CGS 21680 at the highest dose also significantly increased cardiac output (28%) and heart rate (10%) in comparison to occluded vehicle-treated animals. In addition, the highest dose of CGS 21680 significantly reduced Pmcf (9%) and venous resistance (62%) in comparison to occluded vehicle-treated animals. Administration of CGS 21680 did not significantly affect +dP/dt when compared to occluded vehicle-treated animals. 5. The results from the present investigation indicate that occlusion of the coronary artery in rats results in a state of heart failure characterized by reduced arterial pressure and cardiac output, and increased venous resistance, Pmcf and left ventricular end-diastolic pressure. Administration of CGS 21680 to animals with acute heart failure resulted in increased cardiac output which was due to reduced venous resistance, as well as increased heart rate.

Mechanical and electrical properties of smooth muscle cells and their regulations by endothelium-derived factors in the guinea pig coronary artery

Effects of high K+, acetylcholine (ACh), 9,11-epithio-11,12-methanothromboxane A2 (STA2), thrombin and endothelin were investigated on smooth muscles of the guinea pig coronary artery in intact and endothelium-denuded tissues. In intact tissues, ACh transiently inhibited but ATP produced maintained inhibition of the STA2-induced contraction. However, in the endothelium-denuded tissue, ACh produced contraction and ATP inhibited the STA2-induced contraction. In intact tissues, thrombin produced dual actions on the STA2-induced contraction with an initial relaxation followed by contraction. In endothelium-denuded tissues, thrombin enlarged the STA2-induced contraction without transient relaxation. In intact tissues prepared from both proximal and distal regions, endothelin showed the same dual action as observed with thrombin, whereas higher concentrations of endothelin showed only contraction. In endothelium-denuded tissues, endothelin consistently produced contraction. In intact tissues prepared from proximal and distal regions, ACh produced a biphasic response (initial hyperpolarization and subsequently generated depolarization). The amplitudes of both potential changes occurred in a membrane potential-dependent manner. In endothelium-denuded tissues, ACh depolarized the membrane in both tissues. In intact and endothelium denuded tissues, ATP hyperpolarized the membrane in inverse proportion to the membrane potential level, whereas thrombin and endothelin consistently depolarized the membrane. The results indicate that ACh acts on endothelial and smooth muscle cells, and the former releases both EDRF and EDHF. ATP only acts on smooth muscle cells and hyperpolarizes the membrane. STA2, thrombin and endothelin act on both endothelial and smooth muscle cells. STA2 and endothelin may release EDRF but not EDHF, and thrombin may release EDRF and endothelin.

Vessel reactivity and prejunctional modulatory changes in the portal vein of mature spontaneously hypertensive rats

The prejunctional effects of 2-chloroadenosine on the contractile responses to perivascular nerve stimulation were studied in conjunction with vessel reactivity in portal veins from mature (45-60 weeks) spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) controls. It was found that the contractility of the portal veins to exogenous noradrenaline was enhanced in SHR, while the sensitivity to noradrenaline, as seen from the EC50 values, was not altered. Responses to perivascular nerve stimulation (supramaximal voltage, 0.7 ms pulse duration for 10 s) were also enhanced in spontaneously hypertensive rats compared to WKY controls at all frequencies of stimulation tested (2-64 Hz), although the sensitivity to perivascular nerve stimulation was not changed since a response of 50% of the maximal obtainable was achieved at 8 Hz in both SHR and WKY. However, no evidence for change in the level of prejunctional modulation of nerve stimulation responses or postjunctional modulation of noradrenaline responses by 2-chloroadenosine (0.1-100 microM) was found. In conclusion, in mature SHR, responses to perivascular nerve stimulation and exogenous noradrenaline were found to be enhanced compared to responses in WKY, although there was no difference in the modulatory action of 2-chloroadenosine between SHR and WKY.

The effect of ATP analogues on the spontaneous electrical and mechanical activity of rat portal vein longitudinal muscle

The effects of ATP (30-1,000 microM) and four of its structural analogues 2-methylthio ATP (30-1,000 microM), beta,gamma-methylene ATP (30-1,000 microM) (beta,gamma-MeATP), L-beta,gamma-methylene ATP (1-100 microM) (L-beta,gamma-MeATP) and alpha,beta-methylene ATP (0.3-100 microM) (alpha,beta-MeATP) were studied on the spontaneous electrical and mechanical activity of the longitudinal muscle of the rat portal vein using the sucrose gap apparatus. Portal vein preparations displayed spontaneous activity in which slow depolarizations led to bursts of spikes which were concomitant with rhythmic contractions. Low concentrations of ATP analogues initiated an increased frequency of the regular spontaneous waves of depolarizations and contractions. Higher concentrations caused an increased frequency of action potential firing, a sustained depolarization and contraction. The rank order of agonist potency for both contractile and electrophysiological responses was found to be alpha,beta-MeATP greater than L-beta,gamma-MeATP greater than beta,gamma-MeATP greater than 2-methylthio ATP greater than ATP. Prolonged superfusion with alpha,beta-MeATP (10 microM) led to desensitization of P2-purinoceptors so that responses to the other ATP analogues were blocked while responses to noradrenaline were unaffected. On the basis of the rank order of agonist potencies and desensitization with alpha,beta-MeATP, the P2-purinoceptors mediating contraction of the longitudinal muscle of the rat portal vein are classified as being of the P2X-subtype.

ATP causes postjunctional potentiation of noradrenergic contractions in the portal vein of guinea-pig and rat

Superfusion of the portal vein of rat and guinea-pig with Krebs' solution maintained at 25 degrees C greatly inhibits spontaneous contractions of the preparations and allows contractile responses to ATP and noradrenaline to be measured accurately. Under these conditions, continuous superfusion with ATP (10(-5) M), a concentration which had no effect on either basal tension or spontaneous activity, caused a significant shift to the left of the concentration-response curve to exogenous noradrenaline in both tissues. The mechanism of this potentiation induced by ATP may differ in the two tissues since in the rat portal vein potentiation appeared to be rapidly reversed by superfusing with ATP-free solution, whereas in the guinea-pig portal vein a further concentration-response curve to noradrenaline, in the absence of ATP, was still significantly shifted to the left compared with the control curve. However, potentiation in the rat portal vein may have had a longer duration than is suggested by the results since control concentration-response curves to noradrenaline in this tissue showed a progressive shift to the right which, although not significant, is likely to have affected the apparent time course of potentiation. It is concluded that ATP can potentiate contractions to exogenous noradrenaline in the portal vein of rat and guinea-pig via an, as yet, unidentified postjunctional mechanism.

Indirect evidence that purinergic modulation of perivascular adrenergic neurotransmission in the portal vein is a physiological process

The effects of adenine nucleotides and nucleosides on the contractile response to perivascular nerve stimulation were compared in the isolated portal vein of rabbit, rat and guinea-pig. 2-Chloroadenosine was more potent than adenosine and ATP, which were equipotent in producing inhibition of neurogenic contractions in the rabbit and rat via prejunctional P1-purinoceptors. In contrast, neurogenic contractions of the guinea-pig portal vein were not inhibited by adenosine and were potentiated by 2-chloroadenosine and, to a lesser extent, by ATP. Fluorescence histochemical localization of quinacrine, which binds to high levels of ATP, revealed a dense perivascular nerve plexus in the portal vein of rabbit and rat but not of guinea-pig. After chemical sympathectomy, quinacrine-positive nerves persisted in the rabbit (supporting other evidence for the presence of purinergic nerves) but not in the rat (supporting other evidence for ATP as a cotransmitter in adrenergic nerves). It is concluded that a prejunctional purinergic modulatory mechanism operates in adrenergic neurotransmission in the portal vein of rabbit and rat but not guinea-pig, and it is suggested that this indicates a physiological mechanism.

The effect of apamin on non-adrenergic, non-cholinergic vasodilator mechanisms in the intestines of the cat

The effects of apamin, a polypeptide isolated from bee venom, on different vasodilator mechanisms in the small and large intestines were studied in atropinized cats. In the large intestine vasodilatation in response to pelvic nerve stimulation was either abolished or markedly diminished by I.A. apamin. However, neither the contraction of colonic muscle which occurred under these conditions nor sympathetic vasoconstriction was significantly influenced by apamin, suggesting that the effect of the peptide was not a non-specific effect on nerves or vascular smooth muscle. In the small intestine it was observed that the nervous vasodilatation induced by transmural electrical field stimulation or mechanical mucosal stimulation was either diminished or abolished by apamin. Intestinal vasodilatation, caused by close I.A. infusions of 5-hydroxytryptamine (5-HT), was abolished by apamin. After giving apamin 5-HT infusions induced a vasoconstriction in five out of six experiments. Vasodilatation induced by vasoactive intestinal polypeptide (VIP) was not significantly affected by apamin. In a series of in vitro experiments on rat portal vein, dose-response curves of several putative intestinal neurotransmitters were determined in the presence and absence of apamin. The following substances were tested: VIP, substance P, bradykinin, 5-HT, ATP and adenosine. Apamin had no effect on the dose-response curves of any of these compounds. The results are discussed in relation to the possibility that apamin may act by blocking the release of a putative peptidergic transmitter from nerve terminals.

Evidence for purinergic receptors on vascular smooth muscle in rat pancreas

The changes in flow rate in the isolated pancreas of the rat in response to natural purine nucleotides and phosphate-modified analogues were recorded. ATP and ADP transiently decreased the flow rate in a dose-dependent manner but only at very high concentrations (greater than 165 microM). In contrast, alpha,beta-methylene ATP, the most active of the drugs used, decreased the flow rate of the preparation at very low concentrations: 0.495 microM induced about 50% of the maximum effect. beta, gamma-Methylene ATP and AMP-PNP had an intermediary potency. Pyrophosphate had no effect. AMP and adenosine had no vasoconstrictor effect even at 1650 microM. The vasoconstrictor effect of alpha,beta-methylene ATP was not antagonized by phenoxybenzamine (6 microM) or by apamin (0.01 microM). In contrast PIT, a P2-purinergic receptor antagonist, partially or totally blocked the alpha,beta-methylene ATP effect depending on the concentrations used. From our results it can be concluded that purinergic receptors of P2-type are present on the vascular smooth muscle of the pancreatic bed in the rat.

Evidence in support of the P1/P2 purinoceptor hypothesis in the guinea-pig taenia coli

1 The relaxations induced by adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and adenosine on the carbachol-contracted taenia coli of the guinea-pig have been studied. ATP and ADP produce similar responses which differ in nature and time course from those of AMP and adenosine. 2 Theophylline, at concentrations (25-200 muM) lower than those which produce significant phosphodiesterase inhibition, blocks the effects of AMP and adenosine but fails to antagonize the responses elicited by ATP and ADP. The antagonism of adenosine by theophylline appears to be competitive. 3 Apamin (1-100 nM) blocks the inhibitory effects of ATP and ADP but fails to antagonize the responses to AMP and adenosine. The antagonism by apamin is non-competitive. 4 The results indicate that ATP and adenosine relax the taenia coli by activating different receptors and are consistent with the P1, P2 purinoceptor hypothesis.

Inhibition of fundic strips from guinea-pig stomach: the effect of theophylline on responses to adenosine, ATP and intramural nerve stimulation

The effect of theophylline on the ATP response, the adenosine response and the inhibitory junction potential was studied on circular smooth muscle preparations of the guinea-pig stomach. The amplitude of the inhibitory junction potential evoked after stimulation of the non-cholinergic, non-adrenergic nervous system was not affected by a moderate concentration of theophylline (5 x 10(-6)-10(-5) M). At higher concentrations (5 x 10(-5)-10(-3) M) theophylline relaxed the muscle hyperpolarized the cell membrane and reduced the inhibitory junction potential slightly. ATP and adenosine (5 x 10(-6)-10(-3) M) also caused relaxation of the smooth muscle cells and hyperpolarization of the cell membrane. Theophylline (5 x 10(-6)-10(-3) M) did not antagonize these effects; in the presence of theophylline (5 x 10(-5)-10(-3) M) additional relaxations produced by ATP and adenosine were limited in view of the muscle tone. These results indicate that theophylline does not inhibit either the effect of the non-adrenergic inhibitory transmitter on the smooth muscle cells of the guinea-pig stomach or the actions of ATP and adenosine. This suggests that the existence of theophylline-sensitive adenosine receptors in the stomach-muscle cell membrane is unlikely and that theophylline is not the drug of choice to support the purinergic nerve hypothesis.

The response of lymphatic smooth muscles to vasoactive substances

A study was made of the isotonic response of bovine mesenteric lymphatics to several physiological vasoactive substances. Contractions of lymphatic smooth muscles were induced by serotonin (5-HT), prostaglandin F2alpha (PGF2alpha), noradrenaline (NA), histamine, dopamine and acetylcholine (ACh). The smooth muscles were particularly sensitive to 5-HT. Excepting PGF2alpha no other substances could equal 5-HT in the magnitude of the maximum response. The majority of 5-HT receptors seemed to be the D receptors. The decreasing order of the contractile responses was as follows: 5-HT greater than PGF2alpha greater than NA greater than histamine greater than dopamine greater than ACh. The contractile response to ACh was observed only in specimens involving valvular region. It was very likely that, in the lymphatics, there were 2 kinds of receptors for catecholamines, i.e. alpha and beta receptors, and the stimulation of the former induced smooth muscle contraction and that of the latter relaxation. A difference was noticed between the responses of valvular and intervalvular segments to NA. Relaxations of lymphatic smooth muscles were induced not only by isoproterenol but also by adenosine and adenine nucleotides. The decreasing order of the relaxant responses was as follows: ISP greater than adenosine greater than ATP greater than ADP greater cyclic AMP greater than or equal to AMP. The relaxant responses to adenine nucleotides tended to reduce with decrease in the number of high energy phosphates.

Possible feed-back inhibition of noradrenaline release by purine compounds

The contractile responses to transmural stimulation of, and the overflow of tritium from the rat portal vein prelabelled with 3H-noradrenaline were studied. The contractile responses of the rat portal vein were sustained throughout the period of stimulation. The tension developed did not decline when two consecutive periods of stimulation were compared. In contrast, the tritium overflow decreased during the second period of stimulation. Preincubation with 3 micronM phenoxybenzamine during 30 min increased 3-fold the tritium overflow during stimulation. Phentolamine and phenoxybenzamine were nearly equipotent in reducing the vascular response to stimulation. In contrast, phentolamine was less potent than phenoxybenzamine in increasing the 3H-noradrenaline overflow elicited by stimulation. The results obtained with phentolamine are interpreted in terms of a different potency of phentolamine to produce blockade of prejunctional and postjunctional alpha-adrenoceptors in the rat portal vein. ATP inhibited by 70% the tritium overflow induced by stimulation. The potency of ATP in inhibiting the overflow increased when the prejunction alpha-adrenoceptors were blocked. The purine compounds ATP, ADP, AMP and adenosine were roughly equipotent in inhibiting stimulation-induced tritium overflow. The tritium released by stimulation decreased when uptake and metabolism of adenosine were inhibited. Under physiological conditions, a prejunctional purinergic inhibition of noradrenaline release might be involved in an endogenously mediated negative feed-back regulatory mechanism. It is possible that the purinergic inhibition of the noradrenaline liberation elicited by stimulation plays a physiological role in tissues with both purinergic and adrenergic innervation.